As a typical ammonia synthesis method, the Haber-Bosch method uses doubly promoted iron containing several percent by mass of Al2O3 and K2O in Fe3O4 as a catalyst, and passes a mixed gas of nitrogen and hydrogen into contact with the catalyst under high temperature and high pressure conditions to produce ammonia. This technology is widely used industrially in the production process as almost the same as it was completed.
On the other hand, a method of producing ammonia at a temperature lower than the reaction temperature of the Haber-Bosch method has been studied. Catalysts capable of producing ammonia by contacting with nitrogen and hydrogen have been investigated, and transition metals have been studied as their catalytically active components. Among them, a method using ruthenium (Ru) as a catalyst active component on various catalyst supports and using it as a catalyst for ammonia synthesis has been proposed as an efficient method (for example, Patent Document 1).
It is known that a catalyst using a transition metal such as Ru has very high activity and ammonia can be produced under milder conditions than the reaction conditions used in the Harbor-Bosch method. For example, in the Harbor-Bosch method, a reaction temperature of 400° C. or higher and a reaction pressure of 20 MPa or higher are required, whereas with a catalyst using a transition metal, the reaction temperature is 200° C. or higher and 1.1 MPa or lower. It is known that the reaction proceeds even with a reaction pressure around atmospheric pressure.
As another ammonia synthesis catalyst, intermetallic compounds are also being studied. If an intermetallic compound of a transition metal such as Ru having high catalytic activity and another metal element is obtained, it can be expected to be an inexpensive catalyst.
Examples of intermetallic compounds which are active in ammonia synthesis include intermetallic compounds of alkali metals or alkaline earth metals and transition metals such as CaNi5, Mg2Ni and Mg2Cu (Patent Document 2); and intermetallic compounds known as hydrogen storage alloys such as CeFe2, CeCo2 and CeRu2 (Non-patent documents 1 and 2).
Specifically, Non-Patent Document 1 reports investigation results of ammonia synthesis using powders of intermetallic compounds such as CeFe2, CeRu2, CeCo2 and the like prepared by a melting method as catalysts, instead of a simple metal catalyst has been done.
Also a method using a hydride AB5H˜6 obtained by reducing an intermetallic compound represented by an AB5 type intermetallic compound as a catalyst is proposed. Specifically, it has been reported that it is possible to synthesize ammonia at room temperature by using hydride obtained by reducing intermetallic compound represented by the AB5 type intermetallic compound as a catalyst, wherein A is a mischmetal containing La as a main component, B is an intermetallic compound of Ni and has a BET specific surface area of 0.02 m2/g (Non-Patent Document 3).
It is also known that a fine intermetallic compound can be obtained by absorbing hydrogen to be embrittled, crushed, and then removing hydrogen.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2006-23129.
[Patent Document 2] U.S. Pat. No. 4,325,931.
[Non-Patent Document 1] Takeshita, T., Wallace, W. E., Craig, R. S., “Journal of Catalysis” 44, 236-243 (1976).
[Non-Patent Document 2] Wallace, W. E., “Proceedings of an International Symposium Held in Gelio”, Norway, 14-19 August 1977 pages 501-514.
[Non-Patent Document 3] Hai-Yan Zhu, “Journal of Alloys and Compounds” 240(1996) L1-L3.